How Nicotine Works

While there are thousands of chemicals in the tobacco plant (not to mention those added by cigarette manufacturers), one, nicotine, produces all the good feelings that draw people back for another cigarette or plug of tobacco.

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You may have discovered the effects of nicotine in your teen years -- as many as 85 percent of adult American smokers admit they got hooked when they were under 21 -- but tobacco has a more substantial history than sneaking a smoke after school [source: American Lung Association].

Nicotiana tabacum was discovered and then cultivated in the Americas as early as 6000 B.C., and ever since, people have been smoking or chewing the leaves of the plant. Tobacco use was controversial even in its early days. It was believed to have medicinal properties -- tobacco was used to protect against the ravages of the bubonic plague in the Middle Ages -- yet as early as the 1600s, there was speculation that there might be a link between certain diseases such as cancer and tobacco use. Since then, modern research methods have provided evidence of this link; and while there are thousands of chemicals in the tobacco plant (and that's not even including those added by cigarette manufacturers), it's one -- nicotine -- that produces the good feelings that keep users coming back for more.

Tobacco is a member of the deadly nightshade (or belladonna) family of flowering plants, a family that also includes potatoes, tomatoes, eggplant, and capiscum pepper plants -- a family rich in alkaloids. About 5 percent (by weight) of the tobacco plant is nicotine (C10H14N2), a naturally occurring liquid alkaloid. An alkaloid is an organic compound made out of carbon, hydrogen, nitrogen and sometimes oxygen, and it can have potent effects on the human body. The morning cup of coffee you enjoy, for instance, contains another type of alkaloid: caffeine. Cigarettes typically contain about 10 milligrams of nicotine (sometimes up to 20 milligrams, depending on the brand); when you smoke a cigarette, about 1 to 2 milligrams get absorbed by your body [source: PBS].

Let's look at what happens when nicotine and your body get together.

Nicotine in the Body

Just 10 seconds after a cigarette smoker inhales, nicotine is absorbed through the skin and the mucosal linings in the nose, mouth and lungs, and travels through the bloodstream to the brain. It stimulates adrenal glands to produce epinephrine, a hormone and neurotransmitter you also know as adrenaline. This increases heart rate and blood pressure while constricting blood vessels; it also stimulates the production of dopamine, a neurotransmitter that controls the brain's pleasure center. Inhaling nicotine gives the most immediate effects, and that's not a coincidence; it's because your lungs are lined with millions of tiny air sacs called alveoli. Alveoli provide an enormous surface area -- more than 40 times the surface area of your skin -- making it the most efficient way to get nicotine into the bloodstream [source: Richardson]. Nicotine only stays in the human body for a few hours; it has a half-life of about an hour or two, meaning that six hours after smoking a cigarette, only about 0.031 milligram of the 1 milligram of nicotine you inhaled remains [source: Hukkanen, Benowitz].

Nicotine can also be absorbed through your gastrointestinal tract and your skin -- this is how smokeless tobacco products such as chewing tobacco, skin patches and gum deliver their nicotine fix.

Once absorbed by the body, enzymes in the liver break down most of the nicotine -- about 80 percent; here it becomes the metabolite cotinine. Nicotine is also metabolized into cotinine and nicotine oxide by the lungs. Cotinine and other metabolites are excreted in urine, and they're also found in saliva and hair. Cotinine has about a 16-hour half-life, which means if you've smoked in the past day, using this metabolite as a biomarker will give your secret away in a urine screening test [source: CDC].

Effects of Nicotine

Smokers may feel an initial sense of relaxation from cigarette use, but internally, there's a stimulant effect.

Ever wonder why smokers crave a cigarette when they're in a bad mood or in a stressful situation? It's because nicotine may help people feel calmer, causing temporary feelings of relaxation as well as reducing stress, anxiety and even pain [source: World Health Organization].

In spite of that relaxation, though, nicotine actually increases physical stress; its effects are considered a bit of a paradox. It perks up the central nervous system, but depending on the dosage some smokers find nicotine also acts as a sedative. Some studies, though, suggest it may just be the ritual of smoking that induces a calming effect, because nicotine is actually considered a stimulant, not a depressant.

When you first inhale, nicotine causes your body to release the hormone epinephrine, which is the "fight or flight" hormone. Epinephrine activates the sympathetic nervous system, making your breathing become rapid and shallow, your heart rate increase, and your blood pressure rise. You're alert.

Nicotine can also lead to insulin sensitivity and insulin resistance, as well as an increased risk of developing metabolic syndrome, type 2 diabetes and cardiovascular disease. It affects thyroid hormones, pituitary hormones, sex hormones and adrenal hormones. Insulin resistance in cigarette smokers, for example, may be in part because nicotine stimulates the body to produce high levels of adrenal androgens -- in other words, nicotine impacts the body's glucose metabolism, leading to hyperglycemia and hyperglycemia associated with insulin resistance. Insulin resistance not only increases the risk of developing type 2 diabetes but also heart disease [source: Kapoor].

Nicotine and the Brain

A synapse is the site where two neurons come into contact. The presynaptic neuron releases a neurotransmitter, which binds to receptors on the postsynaptic cell. This allows signals to be transmitted from neuron to neuron in the brain. Acetylcholine is released from one neuron and binds to receptors on adjacent neurons.

Let's imagine your brain as a computer. Like a computer, your brain processes, stores and uses information. In a computer, information travels in the form of electricity moving through wires; information transfer is a binary process, with switches being either on or off. In your brain, neurons are the cells that transfer and integrate information. Each neuron receives input from thousands of other neurons throughout the brain, processes that information and makes any necessary adjustments before communicating the message throughout the body. While signals are conducted through individual neurons as electric current, it's a group of chemical messengers called neurotransmitters that actually relay those messages between neurons and other cells [source: National Institute on Drug Abuse].

Each neurotransmitter has its own specific family of receptors. Nicotine happens to imitate the neurotransmitter acetylcholine, and binds to those receptors (specifically those known as the nicotinic receptors). However, unlike acetylcholine, nicotine is not regulated by your body. While neurons typically release small amounts of acetylcholine in a regulated manner, nicotine activates cholinergic neurons (which would normally use acetylcholine to communicate with other neurons) in many different regions throughout your brain simultaneously.

Because of all of that unregulated stimulation and disruption, your body increases its release of acetylcholine, leading to heightened activity in cholinergic pathways throughout your brain. Activity in the cholinergic pathways calls your body and brain into action, and you feel re-energized. Stimulating those cholinergic neurons also increases how much dopamine gets released by the limbic system, which activates reward pathways in your brain. When drugs like cocaine or nicotine activate the reward pathways, it reinforces your desire to use them again because it feels good [source: National Institute on Drug Abuse].

Nicotine also stimulates the release of another neurotransmitter, glutamate; glutamate is involved in learning and memory and enhances the connections between sets of neurons. These stronger connections may be the physical basis of what we know as memory, and when you use nicotine, glutamate may create a memory loop of the good feelings you get and further drive the desire to use nicotine.

Nicotine also increases the level of other neurotransmitters and chemicals that modulate how your brain works. For example, your brain makes more endorphins in response to nicotine. Endorphins are small proteins that are often called the body's natural painkiller. It turns out that the chemical structure of endorphins is very similar to that of heavy-duty synthetic painkillers like morphine. Endorphins can lead to feelings of euphoria, and may explain nicotine's psychoactive and rewarding effects.

Addiction and Withdrawal

On a molecular level, nicotine is made up of carbon, hydrogen, nitrogen and sometimes oxygen.

Nicotine is addictive, as addictive as cocaine or heroin (and also difficult to quit) -- even if your body absorbs only a tenth of the amount of nicotine in every cigarette you smoke -- just 1 milligram out of the 10 an average cigarette contains -- it's enough to get you hooked [source: Balfour, MedicineNet]. Most smokers report they want to quit the habit, but without cessation help, more than 85 percent of those who try to stop will start smoking again in about a week [source: National Institute on Drug Abuse].

Trying to quit your nicotine habit may cause physical reactions such as strong cravings for the substance, an increased appetite, insomnia and sleep disturbances, gastrointestinal problems and mood-related complaints including anxiety, anger and frustration, depression, irritability and restlessness [source: MayoClinic].

As the effects of the nicotine already in your system begin to wear off, your body begins to crave it -- and that could happen within just two hours of your last smoke [source: NIH]. To get the same effect from the drug, you need more of it each time, which leads to a pack-a-day habit and dependence [source: Psychology Today].

You don't need to inhale nicotine to become addicted. Smokeless tobacco products are also addictive; an average-sized dip resting between your cheek and lip for about 30 minutes will give you the same nicotine fix as if you smoked three cigarettes [source: BeTobaccoFree.gov].

A combination of counseling to help with the psychosocial part of quitting and nicotine-replacement therapy is considered the most effective method for successfully quitting a nicotine habit, long term [source: Lande]. Psychological counseling focuses on learning how to overcome habits you associate with smoking, such as associating smoking with driving in your car or smoke breaks at work. Nicotine replacement therapy helps ease the physical effects of withdrawal.

Health Risks: Nicotine's Dark Side

Nicotine has been used as a commercial insecticide and fumigant, and the news doesn't get better. Each year, 440,000 cigarette smokers die, and for every one death caused by a smoking-related disease there are 20 more people living with at least one serious smoking-related illness [source: CDC]. Tobacco use of any kind comes with numerous health risks. Tobacco smoke contains at least 250 chemicals known to be toxic or to cause cancer. A smoking habit of just one to four cigarettes a day is enough to increase your chance of developing cardiovascular disease. Pneumonia, emphysema and respiratory infections, cataracts and eye problems, and certain cancers including cancers of the mouth, pharynx, larynx, esophagus, stomach, pancreas, cervix, kidney, ureter and bladder as well as some leukemias are all known risks of a nicotine addiction. The risk of dying from cancer -- lung or another type -- doubles if you're a smoker, and as many as nine out of 10 people diagnosed with lung cancer can directly blame their tobacco habits [source: MedicineNet, MayoClinic].

Smokers are also more likely to suffer from periodontal disease, diabetes (or insulin resistance), infertility and pregnancy complications, and changes in their physical appearance (including premature aging and yellow-stained, sallow skin). On average, nonsmokers live about a decade longer than smokers [source: CDC].

Despite its dark side, nicotine hasn't always been vilified; Native Americans, for example, smoked tobacco not only for ceremonial reasons but also considered it to have medicinal properties. And it may. Today's research finds there may be some positive uses for nicotine through safer delivery methods than cigarettes, including evidence that it may reduce the risk of developing Parkinson's disease and Alzheimer's disease. It may also show promise as a treatment for depression, anxiety and schizophrenia, and could possibly even help with wound healing [source: Graham, Leary].

Toxicity

While we know smoking tobacco is linked with certain diseases and chronic conditions that will lead to an early death, nicotine is also lethal if ingested in high doses; an oral dose (consumed rather than inhaled) of 50 to 60 milligrams of nicotine is enough to kill a 160-pound person [source: NIOSH]. For example, manufacturers recommend no one chew more than 24 pieces of nicotine gum in one day, because if you were to ingest any more nicotine than that you'd risk an overdose. Yes, you can overdose on nicotine.

Nicotine is a stimulant, as we've discussed, and when you're exposed to too much, it causes the cholinergic neurons at the cholinergic receptors in your body to get excited. It's the same response your body's nervous system has when poisoned by organophosphate insecticides such as DDT, and the same as when you're exposed to nerve agents such as sarin gas. When you're exposed to toxic levels of organophosphate, the neurotransmitter acetylcholine builds up at synapses and disrupts nerve impulses. Acetylcholine is able to imitate the electrical stimulation you'd normally associate with your vagus nerve, and all that excess acetylcholine overstimulates the neurons. Because nicotine is so similar to acetylcholine, it too binds to nicotinic cholinergic receptors and, in excess, produces the same overstimulation -- the more nicotine available in your body to bind to the nicotinic cholinergic receptors, the greater the severity of the poisoning.

Nicotine poisoning delivers a biphasic effect -- it first acts as a stimulant in the body but rapidly turns into a depressant. Vomiting is the most common symptom of nicotine poisoning and can begin as quickly as 15 minutes after ingestion. Nicotine may also cause seizures and involuntary muscle twitching, as well as abnormal heart rhythms, a slow heart rate and fluctuating blood pressure. In high concentrations, nicotine may cause death within as little as an hour, usually due to heart failure, muscle paralysis and a buildup of fluid in the lungs' air passages [source: CDC].

If nicotine poisoning is feared, call a poison control center immediately.

Author's Note: How Nicotine Works

It wasn't the statistics about the health risks of smoking tobacco or how addictive nicotine can be that surprised me as I researched the effects of nicotine; rather, it was that the tobacco plant is in the same family of plants as some of my favorite edibles: potatoes, tomatoes, eggplant and hot peppers. - MT

Royal College of Physicians of London. "Tobacco Advisory Group. Nicotine Addiction in Britain: A Report of the Tobacco Advisory Group of the Royal College of Physicians." Page 1. 2000. (Nov. 24, 2013) http://books.google.com/books?id=y62ZBt538WAC&dq=nicotine%20addiction%20in%20britain